Many electrical and mechanical applications involve measuring relative motion between two objects. This motion may include translation, rotation, or a combination of both. A wide variety of sensing technologies have been developed to accomplish this task.
An important category of such displacement sensors are those that accomplish their measurement in a non-contacting manner. In this category no direct mechanical connection exists between the two objects or between the measuring device, which may be mounted to move with one of the objects, and the other object. The advantages of non-contacting measurement include: zero friction, high immunity to contamination, zero wear, long lifetime, inherent electrical safety, and physical isolation of the two moving objects from one-another. In many cases, mechanical guides or bearings are used to constrain the relative motion of one or both of the objects to desired directions, and these can generally be implemented without compromising the main advantages of the non-contacting sensors themselves.
Non-contacting displacement sensors include optical sensors and magnetic sensors. In magnetic sensors, magnetic field domains are established on one of the objects, either with permanent magnets, or with energized coils. A magnetic flux detector mounted to the opposing object detects the relative motion by sensing the change in magnetic flux.
In optical sensors, one object, called the “target” object, includes regularly spaced marks. Often, multiple tracks of marks are used to provide the ability to distinguish the direction of movement, and some absolute starting point. Mounted to the second object, called the “reference” object, is a light source and an optical detector which can image the marks on the target. By tracking the movement of the marks in the image field, the system can detect and measure relative motion between the target and the reference objects. Such sensors have been configured to measure either translation, with the marks configured along a straight line, or rotation, with the marks configured around the circumference of a circle on a plane, such as a disk or with marks configured around the surface of a cylinder, such as a drum.
With either the magnetic or the optical non-contacting sensors the target object was often engineered with the application of magnetic domains or visible marks. One large field of use for optical displacement sensors that did not require an engineered surface has been optical computer mouse sensors that inherently measure the relative displacement between the reference object, the mouse, and the target object, a surface such as a table top. Optical computer mouse sensors have been specifically designed to allow for the use of non-engineered targets with no marks added to the target surface. Instead the optical mouse sensors use whatever naturally existing visible features are present as the target “marks” to be imaged. Those computer mouse sensors that illuminate the target with coherent laser light have been particularly well adapted to using a wide variety of surface materials as the target. The coherent light reflecting from microscopic asperities present in nearly all surface materials generates a high contrast interference speckle pattern that is ideal for imaging and mouse position detection. Microscopic asperities are unintentionally occurring unevenness or roughness of a surface. They may have a size in the range from the wavelength of light used to hundreds of microns. They may typically have a dimension of a fraction of a micron to tens of microns. They may be randomly distributed along the surface. Each is usually different from others.
Applicants recognized further applications of such non-contacting optical sensors, and these applications are provided by the following description.